This invention relates to information storage systems that use magnetic disk media. More particularly, the invention relates to a disk drive that uses patterned magnetic disk media.
Magnetic recording disks are typically produced by depositing a thin magnetic recording layer on a suitable disk substrate. Data is generally written on the magnetic layer by a recording head that writes magnetic data bits in the magnetic layer while scanning the surface of the disk. To increase the magnetic recording density of the magnetic disk, the bit sizes in the magnetic layer need to be as small as possible.
Magnetic bit volumes in conventional crystalline magnetic media may include hundreds of magnetic grains or sets of grains. Grains and/or sets of grains are often surrounded by segregating materials that separate neighboring grains and reduce exchange-based magnetic coupling. In order to increase the areal density of disk drives the surface accessible dimensions of magnetic bit volumes must be reduced. This can generally be accomplished by reducing the average grain size while maintaining a critical number of grains in each bit volume to obtain adequate signal to noise ratios.
Depositing thinner magnetic layers often results in smaller grains and potentially higher areal densities of disk drives. However, it is well known in the art that a limiting factor for reducing the average grain size in a magnetic recording layer is the onset of superparamagnetism. This situation arises when the magnetic volumes of grains or groups of grains are thermally unstable, either at room temperature or at elevated temperatures. A magnetic layer with a large number of superparamagnetic magnetic grains is incapable of storing magnetic data for long periods of time.
Instead of reducing bit sizes by reducing the grain sizes, it has been suggested that patterning methods can be used to define bit boundaries and increase the bit densities of magnetic media. An early example of patterning magnetic media in a circumferential direction is disclosed in IBM Technical Disclosure Bulletin, Vol. 18 No. 15 October 1975, where a layer of a medium is patterned with circumferential magnetic and non-magnetic tracks alternating at different radial positions of a disk medium. Radial position, herein, denotes a position on a disk medium relative to the center the disk and radial direction refers to a direction that extends either inward or outward and that is substantially normal to a circumferential track. Circumferential position refers to a position along a circumferential track and circumferential direction refers a direction that extends along or follows a circumferential track.
Patterning a magnetic disk medium with magnetic and non-magnetic circumferential tracks results in sharp circumferential boundary definitions that can result in improved magnetic transitions. Further, because the magnetic circumferential tracks are separated by non-magnetic circumferential tracks, edge anomalies and cross-talk between data bits located in adjacent tracks is reduced. Other examples of patterning magnetic layers with circumferential recording tracks are described by Brady et al., in U.S. Pat. No. 5,571,591, and by Krounbi et al., in U.S. Pat. No. 4,935,278.
Patterning magnetic media in two-dimensions to form small isolated magnetic islands has also been described. For example, Fernandez et al. characterize isolated Co magnetic domains in xe2x80x9cMagnetic Force Microscopy of Single-Domain Cobalt Dots Patterned Using Interference Lithographyxe2x80x9d, IEEE Trans. Mag., Vol. 32, pp. 4472-4474, 1996. By using interference lithography to pattern a resist coated silicon wafer followed by thermal evaporation of Co, isolated arrays of magnetic Co domains are generated. Krauss et al. in xe2x80x9cFabrication of Planar Quantum Magnetic Disk Structure Using Electron Beam Lithography, Reactive Ion Etching, and Chemical Mechanical polishingxe2x80x9d J. Vac. Sci. Technol. B 13(6), pp. 2850-2852, Nov/Dec 1995, describe an etching processes to define domains followed by an electroplating step to isolate magnetic Ni domains.
Other methods of two-dimensional patterning of magnetic media are disclosed by Falcone et al., in U.S. Pat. No. 4,948,703. Falcone et al. describe a method of embossing a photo-polymer to pattern the surface of an optical disk and Chou et al., in xe2x80x9cImprint Lithography with 25-Nanometer Resolutionxe2x80x9d, Science, Vol. 275, Apr. 5, 1996, and U.S. Pat. No. 5,772,905 describes a method for embossing PMMA at elevated temperatures and pressures with a template to achieve high resolution patterning. Chou in xe2x80x9cPatterned Magnetic Nanostructures and Quantized Magnetic Disksxe2x80x9d, Proc. IEEE, Vol. 85, No 4, pp. 652-671, April 1997, further describes a method for making magnetic domains with ferromagnetic materials such as cobalt or nickel by electroplating a PMMA embossed surface. The magnetic material fills the depressions in the embossed PMMA surface and creates small magnetic domains.
The Methods of patterning magnetic media with circumferential tracks, described above, fail to take advantage of high-resolution optical techniques for patterning, such as optical interference lithography, which can rapidly pattern an entire disk. This is because optical interference lithography has a technical limitation of primarily being useful to perform patterning with linear dimensions and is an extremely difficult technique to adapt for creating patterns with curvature, such as is needed for circular or circumferential tracks.
One of the shortcomings of using optical interference lithography to pattern a medium in two dimensions is that it requires that the medium be patterned in a two step process. For example, in using optical interference lithography to pattern a magnetic medium, a first patterning step is performed to pattern the medium with a first set of linear patterning lines. Subsequently, in a second patterning step, a second set of linear patterning lines are made on the medium, preferably with the second set of lines extending in a direction orthogonal to the first set of patterning lines. The two step patterning process produces small magnetic islands that define the boundaries of the magnetic bits. Unfortunately, the process of patterning a second set of lines on the surface of a magnetic medium can alter the dimensions of the bits in the direction used during the first patterning step, thus introducing inconsistencies in the patterning process from disk-to-disk. These inconsistencies in bit dimensions from disk-to-disk make it difficult to consistently match the dimensions of a read head and write head to the bit dimensions. Therefore, to achieve good matching between bit and head dimensions, the head and bit sizes of a medium pattered by this method must be individually matched for each magnetic data storage system produced.
There are several advantages to patterning magnetic media. As already mentioned the well-defined boundaries of patterned bits can reduce cross talk between adjacent bits and provide sharper magnetic transition within the bits. Also, there is a potential to greatly increase the areal bit density by reducing the bit surface dimensions. Theoretically, pattering may be used to define very small individual bits that have sufficient magnetic volumes to be thermally stable.
Ideally, a magnetic disk storage system includes a patterned magnetic disk with bits which have patterned boundary dimensions closely and consistently matched with the dimensions of the read and write head. If the bits of the magnetic disk medium have dimensions that are smaller than the dimensions of the read sensor width, signal is lost. On the other hand, if the magnetic disk medium has bit dimensions that are larger than the dimensions of the write pole tip or read sensor width, the storage density of the magnetic medium is not fully utilized. Further, head/bit matching needs to reproducible from data storage system to data storage system.
A third shortcoming of magnetic storage systems and magnetic disk media disclosed in the prior art is that disk media require servo data to be pre-recorded on the disks. The pre-recorded servo data is used in the magnetic storage systems to locate and position read and write devices near surfaces of the magnetic media. Writing magnetic servo-data requires costly equipment and adds a significant amount of time to the manufacturing process of disk media.
What is needed is a magnetic data storage system with patterned magnetic disk medium that has bit dimensions, which are defined by patterning and that are easily matched to the dimensions of a read and write device. The magnetic medium is preferably patterned using high-resolution interference lithographic techniques so that rapid, high-resolution patterning is achieved and so that a mask is not required. It is further desirable that the patterned magnetic medium reduce or eliminate the need to pre-record servo-data on the medium.
Accordingly, it is a primary object of the present invention to provide a magnetic disk medium that is patterned with linear magnetic channels for storing magnetic data. The linear patterning provides for data bits to have sharp edge definitions, which provide for improved magnetic transitions within data bits.
It is a further object of the present invention to provide a magnetic disk medium patterned with magnetic channels extending substantially in a radial direction relative to the center of the disk. This arrangement of linear magnetic channels provides for easy matching of bit dimensions with the dimensions of the read and write head. The matching of the head pole tip dimensions to the bit dimensions in the radial direction maximizes the storage capabilities a disk drive unit and optimizes the data signal generated from data bits within the disk medium. It also allows each disk surface to be formatted optimally for the particular head dedicated to that surface, again providing for maximum compactness of the head/disk combination.
It is a further object of the present invention to provide a magnetic disk medium that is patterned with linear magnetic channels extending in a substantially radial direction and which are patterned by high resolution optical interference lithography. High-resolution optical interference lithography does not require a mask for patterning and, thereby, simplifies the disk manufacturing process.
It is yet another object of the present invention to provide a magnetic disk medium, which is patterned into spatially modulated sections. The spatially modulated sections provide an inherent servo signal that is used in a servo-tracking system for locating and positioning the read and write head. The inherent servo signal generated by the spatial modulation of patterned sections reduces or eliminates the need for writing pre-recorded servo data on the disk medium and thus reduces disk-manufacturing costs.
The disk medium of the current invention enjoys the benefits of a patterned magnetic disk medium and is capable of being used in a variety of disk drive systems with varying radial pole-tip widths.
The objects and advantages of the invention are achieved by providing a disk medium that is patterned with linear channels extending in a radial direction of the disk. The magnetic channels are separated by channel boundaries that prevent or reduce magnetic coupling between the magnetic channels. The linear magnetic channels may be patterned by any patterning method, but are preferably patterned by a photo-lithographic technique such as interference lithography. Interference lithography has an advantage over alternative patterning methods because it does not require a patterning mask.
The magnetic disk medium has a magnetic layer that is preferably a Co alloy magnetic layer 1.0 to 500 nanometers thick and is deposited on a suitable disk substrate. Suitable disk substrates include, but are not limited to, disk substrates of a aluminum-magnesium alloy with a coating of nickel phosphorus, glass, silicon, ceramic or quartz. The magnetic layer is more preferably a Co alloy with Cr in the range of 0 to 30 atomic percent, Fe in the range of 0 to 40 atomic percent and Pt in the range of 0 to 80 atomic percent. Most preferably, the magnetic recording layer also contains B in a range from 0 to less than or equal to 25 atomic percent. The magnetic layer is generally coated with a protective carbon top coat to prevent oxidation and degradation of the magnetic layer.
This invention can be utilized to pattern magnetic disk layers with the easy magnetization axis orientated either in the conventional longitudinal (parallel to the disk) or perpendicular to the disk and can be used to pattern either crystalline or amorphous magnetic layers.
A magnetic disk drive that employs the patterned magnetic medium of the current invention may be connected to a data input unit. The input unit may include any input devices known in the art, such as a computer and/or a microprocessor. The microprocessor, for example, receives input instructions or data through a keyboard a voice activated program or any of the numerous means for inputting data known in the art. The input data is transferred to a disk drive unit with a read and write head, which scans the disk medium in a circumferential direction and writes magnetic data bits in the patterned magnetic channels. The disk drive unit also has a servo positioning system for systematically locating and positioning the read and write head. The servo positioning system reads servo marks positioned at predetermined locations on the disk medium. The servo marks are preferably magnetic servo-marks, but may also be optical servo marks.
In a particular embodiment of the current invention, the magnetic disk medium is patterned into spatially modulated sections. The sections are further patterned with linear magnetic channels separated by channel boundaries. Both the magnetic channels and channel boundaries are parallel to each other within each patterned section. A soft sector pattern is written in two or more channels using the write head to produce magnetic marks that are offset in a radial direction by 50% of the track pitch with respect to the data marks, as shown in FIG. 9. The spatial modulation of the magnetic patterned sections provides a means to generate a servo-signal used by the servo positioning system to locate and position a read and write head during a scanning operation. In addition, the end of one patterned wedge and the beginning of the next can act as a sync mark to allow the drive to know when a new wedge has started. Once that sync mark is detected the drive looks for the soft-sector pattern which is used to generate a feedback signal to control radial positioning of the read and write head.
In the present invention, the data bits within the disk medium are defined in a substantially circumferential direction by the linear edges of the patterned magnetic channels and are defined in a substantially radial direction by a width of the read and write head. In this invention, a write-head refers to any type of head capable of writing marks. In particular, it refers to a ring head or a pole head. In FIG. 5, a ring head is shown, which is useable for both longitudinal or perpendicular recording, but a pole head may also be used for perpendicular recording. The read element is not shown in the figures, but could be a spin-valve, an MR-sensor, a tunnel junction or an inductive read head, for example. It is advantageous to precisely match dimensions of data bits with the dimensions of the read and write head to maximize the storage capabilities of the disk medium. The lithographic processes currently used in the art to define widths of head pole-tips in the radial direction are not as controllable and reproducible as the thin film deposition process that define thicknesses of the pole tips in a circumferential scanning direction.
Matching the circumferential scanning thickness of the pole tip with precise lithographic patterning of magnetic channels in the radial direction can be accomplished consistently. By defining the track pitch with the less well controlled radial read and write head widths, each head/disk can be independently formatted with a different track pitch. In this way the magnetic storage system of the current invention enjoys the benefits of a patterned magnetic disk medium and overcomes the difficulties associated with matching bit dimensions to the dimensions of pole-tips.